High efficiency double suction impeller

ABSTRACT

A double suction impeller is disclosed. In at least one embodiment, the impeller is configured for centrifugal pumps and hydraulic power recovery turbines. The impeller&#39;s flow-path arrangement comprises inter-blade channels, intersecting each other at the impeller&#39;s outer diameter and defining a variable cross section shape, so that the equivalent number of blades is at least doubled with respect to a conventional configuration obtained by the coupling of two single suction impellers and an improved control over the velocity of the flow within the inter-blade channels is achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending InternationalApplication No. PCT/EP2017/078356, filed Nov. 6, 2017, entitled “HighEfficiency Double Suction Impeller,” which claims priority to ItalianApplication No. 102016000111763, filed Nov. 7, 2016, also entitled “HighEfficiency Double Suction Impeller,” each of which is hereby expresslyincorporated by reference as part of the present disclosure.

BACKGROUND

The present disclosure relates to a high efficiency double suctionimpeller, e.g., one that may be used for centrifugal pumps.

Radial flow turbo machinery devices are particularly adapted to convertshaft power to kinetic energy (and vice versa) by accelerating (ordecelerating) a fluid in a revolving device called impeller. When usedas power-absorbing machines, impellers are commonly used to raise thepressure of a fluid or induce a fluid flow in a piping system.

The impeller is the device, within the turbo machinery, that, rotating,exchanges energy with the fluid. In its simplest implementation, theimpeller comprises a plurality of blades fitted onto a hub plate. Theshape and the geometry of impeller blades can be of many different typesdepending on the use, the rating, the performance of the turbomachinery.

Having defined the specific speed, NS, of a pump as follows:

${NS} = \frac{{nQ}^{1/2}}{1.1618H^{3/4}}$

where,

-   n=rotating speed in revolution per minute-   Q=volumetric flow rate in [m³/h]-   H=differential head [m]    for centrifugal pumps of capacity larger than 10 m{circumflex over    ( )}3/h designed with low or medium specific speed values (e.g.,    NS<1600) and a double suction configuration, an impeller with a    small number of blades is required in order to keep the head vs flow    rate stable and continuously rising towards zero flow. This    requirement is very important especially in case of more than one    centrifugal pump employed in parallel, each working with a fraction    of the available flow. Furthermore, centrifugal pumps of large    capacity designed for low or medium specific speed values and for    medium or high values of hydraulic head, require impellers having    large diameters and narrow exit width. Double suction impellers are    usually composed by two single suction impellers each elaborating    half of the total flow and arranged in a back-to-back configuration.

In the state-of-the-art, centrifugal pumps having impellers providedwith a center rib and staggered blades, the ratio between the impellerexit width b2 and the impeller diameter D2 can be well lower than 0.05.Impellers of this kind often show an unstable head vs flow ratecharacteristic curve. In addition to that, another drawback of this kindof impellers lies in the low blade exit angles (normally between 15° and20°) and corresponding large wrap angles (normally between 120° and270°) that are required to maintain acceptable slip factor values. As aresult, the hydraulic efficiency of the state-of-the-art impeller ofthis kind is typically smaller than 95%.

Furthermore, the low blade load typical of this kind of impellers(normally corresponding to head coefficients “psi” lower than 1, psibeing equal to:

${psi} = \frac{2{gH}}{u_{2}^{2}}$

where

-   g=gravity acceleration in [m/s{circumflex over ( )}2]-   H=differential head in [m]-   u2=peripheral speed of the impeller in [m/s]) increases the required    diameter of the impeller, thus increasing the disk friction losses    by 1%-2% when compared to impellers having head coefficients greater    than 1.

The achievable head coefficient can be increased by employingconventional split blades impellers, but this choice does not solve theproblem of the narrow b2/D2 and poor head curve stability. Moreover, thenumber of leading edges of conventional split blades impellers isdoubled, causing additional hydraulic losses.

SUMMARY

Embodiments of the present invention therefore relate to a doublesuction impeller having the channels between the blades starting fromboth inlets and crossing the median axis of the impeller exit in such away that, as a result, the equivalent blades number is doubled withrespect to a conventional configuration obtained by the coupling of twosingle suction impellers.

In the new impeller, the reduction of the slip by increasing theequivalent number of blades permits to reduce the diameter of theimpeller, thus reducing the size and therefore the manufacturing cost ofthe pump installing said impeller.

Compared to a conventional impeller provided with splitter blades,embodiments of the new impeller do not introduce any additional leadingedge and corresponding losses.

For low specific speed pumps the new shape of the inter-blade channelsof the impeller is such that the hydraulic diameter is increased and thelength of each channel reduced, thus reducing the hydraulic losses withrespect conventional impellers.

The cross section area of the inter-blade channels of the impeller isdesigned to have control over the velocity of the flow within theinter-blade channel. Therefore, the shape of the inter-blade channels isadapted to avoid that the inter-blade channels of the impeller intersectwith each other and at the same time to have a cross-sectional area thatchanges its shape along the flow path.

Furthermore, the reduction of the impeller diameter brings also along asignificant reduction of the losses due to disk friction, thusincreasing the overall pump efficiency.

The advantages and benefits associated to the new double suctionimpeller, with respect to an equivalent state-of-the-art impeller,increase as the operating speed of the new double suction impeller isdecreased.

The impeller may be shrouded or unshrouded, and it can be made of onesingle piece or made of two pieces merged where the blades coming fromopposite side adjoin.

The main application for the new double suction impeller is withincentrifugal pumps, pump as turbine (PAT) and hydraulic power recoveryturbines (HPRT) especially, but not exclusively, intended for refinery,petrochemical and pipelines. However, other applications are possibleand contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of embodiments of thepresent invention will become better understood when the followingdetailed description is read with reference to the accompanying drawingsin which like characters represent like parts throughout the drawings,wherein:

FIG. 1 illustrates a meridional section of a state of the art doublesuction impeller;

FIG. 2 illustrates a view of the inter-blade channel of the state of theart double suction impeller;

FIG. 3 illustrates a detail of the outer diameter arrangement of thestate of the art double suction impeller;

FIG. 4 illustrates a detail of the section of an embodiment of a newdouble suction impeller according to the present disclosure;

FIG. 5 illustrates a view of the inter-blade channel of anotherembodiment of the new double suction impeller according to the presentdisclosure;

FIG. 6 illustrates a detail of the outer diameter arrangement of anotherembodiment of the new double suction impeller according to the presentdisclosure;

FIG. 7 illustrates a first front view of another embodiment of the newdouble suction impeller according to the present disclosure;

FIG. 8 illustrates a meridional cross section of an embodiment of thenew double suction impeller according to the present disclosure;

FIG. 9 illustrates a side view of an embodiment of the new doublesuction impeller according to the present disclosure;

FIG. 10 illustrates a graph plotting the efficiency and the headcoefficient vs the flow coefficient of the new double suction impelleraccording to the present disclosure compared to state-of-the-artimpellers; and

FIG. 11 illustrates a view of the inter-blade channel of anotherembodiment of the new double suction impeller according to the presentdisclosure.

DETAILED DESCRIPTION

With reference to the attached drawings and according to an exemplaryembodiment, embodiments of the present invention relate to new a doublesuction impeller, in particular for centrifugal pumps, wherein theflow-path arrangement is characterized by inter-blade channelsintersecting each other at the impeller outer diameter.

In one embodiment, the new double suction impeller comprises channelsbetween the blades starting from both inlets and intersecting at theouter diameter of the impeller in such a way that the equivalent bladenumber is doubled with respect to a conventional configuration obtainedby simply adjoining to a central rib two single suction impellers, asillustrated in FIGS. 1, 2 and 3.

In greater detail, and with reference to FIGS. 4, 5, 6, 7, 8, 9 and 11,the new double suction impeller includes a shrouded impeller 10. Theshrouded impeller 10 may further include a hub 11 associated with atubular center bore 12. The tubular center bore 12 may be adapted toreceive the impeller drive shaft which is drivingly connected thereto,generally by a key and a keyway.

The shrouded impeller 10 can be made either of one single piece—orassembly—or it can be made of a plurality of assemblies, e.g. comprisingone left shroud, one right shroud and a central core.

In one embodiment, the new impeller is made of one single assembly, andthe hub 11 further includes a plurality of blades 13 integrally attachedto the hub 11 and to a pair of integral shrouds, a left side shroud 14and a right side shroud 15. Each one of the integral shrouds 14, 15 isprovided with a center aperture 16, 31 that constitutes the impellereye. The impeller eye is adjacent to said tubular center bore 12 andcomprises an aperture edge 17 with an aperture edge radius and anaperture rim 18 with an aperture rim radius.

The left side shroud 14 defines the left side aperture and the rightside shroud 15 defines the right side aperture of the double suctionimpeller according to the present invention.

The outer edge of said left side shroud 14 and the outer edge of saidright side shroud 15 define the impeller exit, said impeller exit havinga width 19 and a median plane 20.

In greater detail, each pair of adjacent blades 13 of said plurality ofblades 13 define a plurality of inter-blade channels, referred to inFIGS. 5 and 11. Said inter-blade channels are adapted to connect aplurality of input apertures, located within the center apertures ofboth said left side shroud 14 and said right side shroud 15, to aplurality of output apertures located on said impeller exit.

With reference, in particular, to FIGS. 4 and 6, said inter-bladechannels comprise left side inter-blade channels 21, having theirrespective input apertures 22 located within the center aperture of saidleft side shroud 14, and right side inter-blade channels 24, havingtheir respective input apertures 25 located within the center apertureof said right side shroud 15.

Advantageously, said left side inter-blade channels 21 and said rightside inter-blade channels 24 are such as intersecting the median plane20 of said impeller exit in a way to dispose the output apertures 23 ofsaid left side inter-blade channels 21 to be aligned and alternated withthe output apertures 26 of said right side inter-blade channels 24, onsaid impeller exit.

In operation, when rotated, fluid will be drawn axially into theimpeller as indicated by the arrows 27, 28, impelled by the plurality ofblades 13 passing between the hub 11 and said left and right shrouds 14,15 and finally expelled radially through said exit as indicated by thearrows 29. The impeller runs in the direction of arrow 30 in a suitablehousing having axial inlets and a circumferential volute or diffuseroutlet passage.

In the double suction impeller according to the present invention, theeffect connected to the crossing by said inter-blade channels of themedian axis of the impeller exit is such that the equivalent bladesnumber is doubled with respect to a conventional configuration obtainedby the coupling of two single suction impellers.

With reference to FIG. 2, the section 33 corresponds to the inlet of achannel of an impeller of the state of the art, and area 32 correspondsto the outlet of a channel of an impeller of the state of the art.

With reference to FIGS. 5 and 11, the section labeled 35 corresponds tothe inlet of a channel of the new impeller, and the area labeled 34corresponds to the outlet of a channel of the new impeller.

It is apparent that, when comparing inter-blade channels of a doublesuction impeller of the prior art with the inter-blade channels of thenew double suction impeller, the outlet area 34 of the inter-bladechannels of the new double suction impeller has a rectangular shape withan aspect ratio much closer to 1 with respect to that of a state of theart impeller.

The cross section area of the inter-blade channel is designed to havecontrol over the velocity of the flow inside the inter-blade channel.Moreover, the shape of the inter-blade channel is such as to avoidintersections between opposing channels and at the same time to maintaina target cross section area.

The shape of the new inter-blade channel is adapted to ensure a suitabledistribution of the velocity inside the channel and to avoid channelsintersection. Furthermore, the shape of the new inter-blade channel issuch as the area of the cross section of the channel changes gradually,allowing for a precise control of the fluid velocity inside. Thisfeature leads to higher performance and higher overall efficiencycompared to solutions where the area of the internal section of thechannel changes abruptly in order to avoid mutual intersection betweenchannels or solutions where the area of the internal section of thechannel is kept constant. In a preferred embodiment the cross sectionarea of the channel is such as it allows the velocity of the fluidinside the channel being described by a function continuous in its firstand second derivative. As a non-limiting example, such functions can bethose represented by Bézier curves.

According to another embodiment, shown in FIG. 11, the inter-bladechannels of the double suction impeller have a variable cross sectionshape. The channel starts at the inlet having an essentiallyquadrilateral cross-section. Then, moving along the flow toward theoutlet, the channel cross-section becomes a five sided polygon until asmall length before the impeller outlet where the channel cross-sectionchanges back to being substantially quadrilateral. The additional fifthside defines the channel cross section between the suction side and thehub surfaces of the channel. The length of the additional fifth side ofthe channel cross section starts from zero, increases until it reachesits maximum length and then decreases back to zero.

Finally, benefits introduced by the new double suction impeller include,inter alia, reduction of the slip and reduction of hydraulic losses.These reductions may translate to an efficiency increase of about 3% to4% and, ultimately, to a lower operating expenditure and lower capitalexpenditure.

The above description of exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims.

Throughout the specification, the terms “one embodiment” or “anembodiment” indicate that a particular feature, structure, orcharacteristic described in connection with an embodiment is included inat least one embodiment of the subject matter disclosed. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification is not necessarily referringto the same embodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments. The above detailed description does not limit the scope ofthe claimed invention. Instead, the scope of the invention is defined bythe appended claims.

1. A double suction impeller, comprising: a hub associated to a tubularcenter bore, the hub comprising: a plurality of blades attached to thehub and to a left side shroud and to a right side shroud, the left sideshroud and the right side shroud having a center aperture adjacent tothe tubular center bore, the outer edge of the left side shroud and theouter edge of the right side shroud define an impeller exit, theimpeller exit having a width and a median plane; each pair of adjacentblades of the plurality of blades defines a plurality of inter-bladechannels configured to connect a plurality of input apertures locatedwithin the center apertures of both the left side shroud and the rightside shroud to a plurality of output apertures located on the impellerexit; the inter-blade channels comprising left side inter-blade channelshaving their respective input apertures located within the centeraperture of the left side shroud, and right side inter-blade channelshaving their respective input apertures located within the centeraperture of the right side shroud, wherein the left side inter-bladechannels and the right side inter-blade channels intersect the medianplane of the impeller exit in a configuration where the plurality ofoutput apertures of the left side inter-blade channels are inalternating alignment with the output apertures of the right sideinter-blade channels on the impeller exit, and the inter-blade channelshave a variable cross section shape, the variable cross section shapebeing substantially quadrilateral at the inlet of the inter-bladechannel, then becoming substantially a five sided polygon, and thenbecoming substantially quadrilateral again before the impeller outlet.2. The double suction impeller according to claim 1, wherein theinter-blade channels are configured to enable the velocity of the fluidinside the channels being described by a function whose first and secondderivatives are continuous.
 3. The double suction impeller according toclaim 1, wherein the double suction impeller is made of a single piece.4. The double suction impeller according to claim 1, wherein the doublesuction impeller is made of two pieces merged where the blades comingfrom opposite sides adjoin.
 5. The double suction impeller according toclaim 1, wherein the center bore is configured to receive the impellerdrive shaft, the impeller drive shaft being drivingly connected to saidcenter bore by a key and a keyway.
 6. The double suction impelleraccording to claim 1, wherein the center aperture) comprises an apertureedge and an aperture rim.
 7. The double suction impeller according toclaim 1, further comprising a housing having axial inlets and acircumferential volute or diffuser outlet passage.
 8. The double suctionimpeller according to claim 1, wherein the plurality of blades areintegrally attached to the hub.
 9. A centrifugal pump comprising adouble suction impeller according to claim
 1. 10. A hydraulic powerrecovery turbine comprising a double suction impeller according to claim1.